Astronomy buffs in Taiwan can see on Friday a rare “blue moon,” which refers to the appearance of a second full moon in a calendar month and a saying that dates back to the American Farmers’ Almanac, the Taipei Astronomical Museum said.
The moonrise, which will take place at 6:23 p.m., will be the second full moon in July after a first one that came on July 2, the museum said.
People will be able to easily spot the moon by looking up to the lower eastern sky some time after 6:30 p.m., said museum researcher Chang Kuei-lan.
Just don’t expect it to be “blue,” Chang said, noting that it will look like any other full moon.
According to Chang, the modern usage of “blue moon” can be attributed to a Sky & Telescope Magazine misinterpretation.
In an article published by the astronomy magazine in 1946, it mistakenly defined a blue moon as the second full moon in a calendar month, which is how it is understood today, Chang said.
The magazine said the Earth usually experiences 12 full moons each year, one in each calendar month, but because a lunar month averages about 29.53 days, the extra days accumulated throughout the year will eventually result in 13 full moons in some years.
That was a misinterpretation of the Maine Farmers’ Almanac, which began listing the dates of forthcoming blue moons in 1819 and was the origin of the phrase “once in a blue moon.”